Electric blasting initiator



p 9,. L. A. BURROWS I 2,173,270

ELECTRIC BLASI'ING INITIATOR Filed April 14, 1957 L awzon A. BUFFOufiflNVENTOR.

ATTORNEY.

?atented Sept. 1%, 193% names STATES Zifiilt PA EENT QFFHQE ELECTRIC BLASTENG KNITKA'EGR Application April 14, 1937,Serial No. 136,719

4 Claims.

' This invention relates to electric blasting initiators generally, and more particularly to electric blasting caps without appreciable time lag and substantially free from susceptibility to static 5 electricity effects.

The electric blasting caps in general use comprise a cylindrical metal shell containing a base charge of a detonating explosive and an ignition charge of a more readily ignitible explosive above.

the base charge, frequently with an intermediate charge separating the two. At the time of firing, the ignition charge is ignited by the passage of an electric current through a small diameter, highresistance bridge wire which becomes heated to incandescence by the passage of the current therethrough.

The time lag, or interval of time between the breaking of the bridge wire and the actual instant of detonation of the cap charge is generally known as the induction period" and will be so referred to throughout the present application; other conditions being equal, this serves as a definite characteristic of a particular ignition composition.

In seismographic work, for example, it is desirable to have an initiator with no induction period, or at least as small a one as possible. 7 To further this end, the so-called "fast ignition agents should be employed, namely ones having a negligible induction period. Unfortunately, such compositions are sensitive to static electricity. Therefore, their use would be undesirable in the static susceptible electric blasting cap assemblies known to the art. for the resulting initiator would be unsafe. Consequently, the art is in need of a I fast electric blasting initiator free from susceptibility'to static charges. I

. The object of the present invention .is a new and improved electric blasting initiator which is 40 substantially free from electrostatic susceptibility. A further object is an electric blasting cap of the foregoing nature free from any appreciable induction period. A still further object is an electric blasting cap containing an improved ignition composition possessing extremely rapid or lead salts, and by providing said initiator with internal means for substantially reducing or eliminating the electrostatic susceptibility of said initiator.

While I have disclosed the use generally as 5 ignition compositions of double salts of metal hypophosphites with oxygen-supplying salts of metals, preferably I use the double salts of the hypophosphites of lead and mercury with the salts of nitric acid, chlorlc acid, or perchloric l0 acid. For example, I may employ as such composition mercurous nitrato-hypophosphite, HgNO3Hg(H2PO2); lead nitrato-hypophosphite, Pb(NO3)2-Pb(H2PO2)2 or Pb(NOz)- (HsPOz) lead chlorate hypophosphite, Pb(ClO3)2 15 Pb(H2PO2)z; lead perchlorate-hypophosphite, Pb(ClO4) 2Pb(H2'PO2)2; and the like. My preferred composition comprises the double salt of lead nitrate and lead hypophosphite,-since this possesses the desirable qualities of substantial 20 freedom from induction period, better handling properties, and ready availability of starting materials.

In any electric blasting initiator containing my fast ignition compositions as disclosed in the fore 25 going, it is desirable to.provide the firing circuit with a means whereby the susceptibility to static electricity is substantially eliminated, or greatly reduced. Several means of this type are disclosed and claimed in the copen'ding application of 30 Aughey, Burrows and Lawson, Serial No. 47,385 filed October 30, 1935. I prefer to employ a means which permits 'any static charge to pass harmlessly to the shell wall at a point outside the locus of the ignition composition, which means 35 may be one of two general types. In the first place, the static electricity may be caused to pass to the shell wall by forming an actual physical conducting connection for electricity between the shell wall and one of the leg wires at a point out- 40 side the locus of the ignition composition. In the second place, the static electricity may be caused to pass to. the shell wall by forming a spark gap or arc-between the shell wall and at least one of the leg wires at a point outside the locus of the static-susceptible ignition composition, such that the resistance across the are provided is substantially less than that of the arc formed by the firing circuit and the shell wall at the locus of the ignition composition. In either case, the static electricity will pass harmlessly to the shell wall, where it will be effectively eliminated. A preferred method for carrying out the second type of procedure consists in bending both leg wires of the firing circuit in toward one another at a point above the locus of the ignition composition.

The lead nitrate-hypophosphite, and other compounds of the type disclosed, may be employed as ignition compositions in all types of static impervious electric blasting initiators, including electric squibs, electric blasting caps, and

. delay electric blasting caps, whether the initiating means is of the bridge plug, the concave plug, or the match-head type. A special type in which my compositions are particularly advantageous not only of being very rapid and without ap- V preciable induction period, but also of being substantially free from susceptibility to static. The invention, therefore, makes it possible to produce a safe, reliable electric blasting cap suitable for certain special uses, where the speed of the detonator is one of the primary considerations.

In order to disclose my invention more fully,

reference is made to the accompanying drawing which illustrates several .embodiments. It is to be understood, however, that this is to be taken solely by way of illustration, and is not to be regarded as a limitation upon the scope of my invention, which has many important applications other than those herein particularly illustrated.

Referring generally to the drawing, Figures 1, 2, 3, and 4 represent respectively vertical sections of the usual bridge plug type, a concave plug type, a match-head type of electric blasting cap made in accordance with our invention, and a special bridge plug type including an ignition bead. Similar characters are used throughout the several views and inthe accompanying description to indicate corresponding parts.

Referring now particularly to Figure 1, the blasting cap illustrated comprises a capsule or shell I of a suitable electrically-conducting material, such as a metal, as for example copper. aluminum, and the like. In the base of the shell I is placed a base charge 2 consisting of 80% mercury fulminate and 20% potassium chlorate, and an ignition composition 4 comprising lead nitrato-hypophosphite. The latter surrounds the bridge wire 8, attached to the ends of the leg wires 6 and I. These wires are held in fixed space relationship by means of, the bridge plug 5. The cap is closed by means of the waterproofing composition 9 and the sulfur seal III. The leg wire I is provided with an elbow I I which forms with, the shell wall I an arc, the resistance of which is substantially less than that of the are formed by the shell wall I and the firing circuit at the locus of the ignition composition 4. If desired, the elbow may contact the shell wall and form an electrical connection therewith.

In the electric blasting cap as illustrated in Figure l, charges of static electricity which may be inadvertently applied to the shell wall I or the leg wires 6 and I, will be harmlessly displate, and the bridge wire 8 forms the connecin the field. 10

Referring now to Figure 2, the base charge 2 consists of tetryl, and the priming charge 3 of lead azide. The bridge wire 8, connecting the ends of the leg wires Ii and I ;is surrounded by an ignition composition comprising the double salt of lead nitrate and lead hypophosphite,

gelatinized with collodion. The leg wires are held in fixed space relationship by the concave plug I3, and the cap is sealed by the waterproof composition 9 and the sulfur seal I0. Afiixed to one of the wires I by suitable means, as for example by solder, is a short length of wire I4 which forms an electrically conducting connection between the shell wall I and the leg wire I.

By this means the blasting cap will be effective- 5 1y protected from inadvertent firing by static electricity, since the charge will pass to the shell wall through the connection I4, and thence to the ground without affecting the ignition composition 4.

In Figure 3, the match-head type of electric blasting cap illustrated comprises a detonating base charge 2 of tetryl, a priming charge 3 of lead azide and a match-head flash or ignition composition 4 comprising lead nitrato-hypophcsphite. Disposed within the paper cylinder I8 is the match-head, which comprises two metal plates or foils separated by a strip of insulating material I5. One leg wire is soldered to each tion between the foils. In the diagram, only the forward plate I4 is represented. The blasting cap is closed with the waterproof composition 9 and the sulfur seal I 0.

In Figure 3, the plates or foils of the matchhead, as, for example the forward foil l4, are provided with long handles or tabs I6 and rather broad shoulders II, which extend substantially to the paper tube I8, which is in contact with the shell wall I. The shell wall I and the shoulders II will therefore form a low-resistance are or spark gap which is well outside the locus of the ignition composition 4. The static charge will therefore jump across this arc without affecting the ignition composition 4. It is, of course, essential that the ignition composition 4 should cover only the lower part of tab I6 of the matchhead, and should not be disposed between the shoulders I1 and the shell wall I. In shape, the match-head shown is somewhat different from the conventional design in that the shoulders I! are appreciably wider and the tab or handle I6'is somewhat longer.

As an additional precaution against static, the electric blasting cap as illustrated in Figure 3 is provided with a plug of electrically conducting material I9 which engages the leg wire 6 and the shell wall I in electrically conducting, relationship. The plug I9 is separated from the leg wire 1 by the insulation 20. If desired, the 13mg I9 may consist of a low-melting metal or alloy, as

for example Babbitt metal, which is poured into the cap about the dielectric material 20 surrounding the leg wire 'I. The plug I9 may also consist of compressed graphite or other electrically conducting material, if so desired.

In Figure 4, a bridge plug type of ignition means is shown, where the leg Wires 6 and l are held firmly in place by the bridge plug 5. The ignition composition is in the form of a bead 4 cemented about the bridge wire 8. The double salt of lead nitrate and lead hypophosphite, which comprises the bead, is so rapid in firing and in velocity pick-up that there is no measurable interval between the rupture of the bridge wire and the detonation of the cap. The firing of the ignition charge 4 causes the detonation of the primary charge of lead azide 3, and of the base charge of tetryl 2. A particularly desirable feature for protection against electrostatic effects is shown in this Figure 4, in that the wires 6 and I are bent in toward one another at a point below the place of emergence from the plug and removed from the locus of the bead By this arrangement, any discharge of electrostatic charges takes place at a point well outside the locus of the ignition composition.

The compounds cited in my invention possess advantages over most explosive compounds that have been used, when applied in the form of the head 4 of Figure 4, in that no coating of the bead with a surface layer of a hard pressure-resisting, confining medium is necessary, in order that instantaneous firing may result.

The ignition compositions employed may be prepared by any suitable method. The lead ni- -trato-hypophosphite may be preparedgfor example, by introducing lead hypophosphite into a boiling solution of lead nitrate until solution is brought about. The double salt is then obtained by hot filtration, controlled cooling, filtration of the separated crystals, and drying.

It will be understood also that any of the cap charges of the prior art may be used in the carrying out of my invention. For example, the primary charge may consist of any of those capable of use, such as lead azide, mercury fulminate, diazo-dinitrophenol, nitromannite, and the like. Likewise, as base charge, I may use tetryl, pentaerythritol tetranitrate, trinitrotolu- 8111:, picric acid, mannitol hexanitrate, and the li e.

My invention has been described in detail in the foregoing, but it will be apparent that many variations in construction, compositions, and methods of application may be made without departing from the scope of the invention. I wish to be limited, therefore, only by the following patent claims.

I claim:

1. An electric blasting cap of the bridge plug bead type comprising a charged metal shell, a firing circuit, and an ignition composition comprising a double salt of lead hypophosphite with lead nitrate, the bridge wire of said firing o-ircuit being embodied in a bead of said ignition composition, said firing circuit being provided with a discharging means in'that each leg wire of said firing circuit is bent away from the shell wall at a point between the place of emergence of said wire from the plug material and the locus of the ignition bead, permitting the harmless discharge of static electricity to the shell wall at a point outside the locus of said ignition bead.

2. An electric blasting cap of the bridge plug bead type comprising a charged metal shell, a firing circuit, and an ignition composition comprising a complex salt of a metal hypophosphite with a metal salt of an inorganic oxygen-supplying acid, the bridge wire of said firing circuit being embedded in a bead of said ignition composition, said firing circuit being provided with a dscharging means in that each leg wire of said firing circuit is bent away from the shell wall at a point between the place of emergence of said wire from the plug material and the locus of the ignition bead, permitting the harmless discharge of static electricity to the shell wall at a point outside the locus of said ignition bead.

3. An electric blasting initiator comprising a charged metal shell, a firing circuit assembly including a plug and leg wires, and an ignition composiiton which is susceptible to static electricity, said firing circuit being provided with a discharge means in that each leg wire of said firing circuit is bent away from the shell wall at a point between the place of emergence of said wire from the plug material and the locus of the ignition composition, permitting the harmless discharge of static electricity to the shell wall at a point outside the locus of said ignition composition.

4. An electric blasting initiator comprising a charged metal shell, 2. firing circuit assembly including leg wires and a plug, and an ignition composition comprising a complex salt of an inorganic oxygen-supplying acid, said firing circuit being provided with discharge means in that each leg wire of the said firing circuit is bent away from-the shell wall at a point between the place of emergence of said wire from the plug material and the locus of said ignition composition, permitting the harmless discharge of static electricity to the shell wall at a point outside the locus of said ignition composition.

LAWTON A. BURROWS. 

